This invention relates to terrestrial optical communication, and more particularly to a new and improved all-optical terrestrial optical communication network which integrates both fiber and free-space links without requiring electro-optical conversion between the fiber and free-space links, and which achieves a relatively good link power margin for reliable communication in adverse atmospheric conditions, which seamlessly integrates with long-haul fiber backbone links, which provides safety against unintended eye injuries, and which is implemented with relative convenience and cost-effectiveness.
Modern society requires that enormous amounts of information be transmitted between users in a relatively error-free manner. Most of the information is communicated as digital information, primarily because digital techniques allow more information to be communicated quickly and reliably, and because a significant amount of the information is transferred between computers. The use of computers and the evolution of computer technology is responsible for much of the increased demand for information communication. The demand for information communication has increased dramatically during the past few years and is expected to continue well into the future.
The typical medium which carries a significant amount of the information is electrical conductors or copper wires. The telephone system, having been installed for many years, is the primary media used for local or localized communications. Using wired media for telecommunications and high speed data communications creates difficulties, and these difficulties arise because of the wires. Electrical wires introduce a practical limit to the physical length or distance over which the information can travel. Lengthy conductors attenuate the signals to the point where the recognition of signals becomes difficult or impossible. Signals conducted over the wires also have a finite limit to the signaling frequency and hence the amount of information which they can carry. Furthermore, noise is relatively easily picked up or induced into the wires, and the noise tends to corrupt the signals carried by the wires. Wire conductor media is also difficult or impossible to install in many situations. Some metropolitan areas simply have no available space to accommodate the additional conductors within utility conduits, and gaining access to buildings and right-of-way to install the conductors is usually difficult or impossible and is certainly costly. For these and other reasons, many of the advancements in communications have focused on wireless media for communicating information.
Radio frequency (RF) transmissions avoid many of the physical problems associated with wired media. The atmosphere becomes the medium for the RF communications, and thus physical limitations associated with access, space, and right-of-way are no longer paramount problems. However, because the atmosphere is freely available for use by all authorized users, the possibility of interference is always present. Various techniques have been devised to minimize RF interference, but those techniques are relatively expensive to implement. Furthermore, even those techniques are not effective to assure that enormous amounts of information can be communicated reliably through RF broadcasts, simply because the information is broadcast and can not be confined to secure communication channels or links which could eliminate sources of interference.
Optical media offers many advantages compared to wired and RF media. Large amounts of information can be encoded into optical signals, and the optical signals are not subject to many of the interference and noise problems that adversely influence wired electrical communications and RF broadcasts. Furthermore, optical techniques are theoretically capable of encoding up to three orders of magnitude more information than can be practically encoded onto wired electrical or broadcast RF communications, thus offering the advantage of carrying much more information.
Fiber optics are the most prevalent type of conductors used to carry optical signals. Although the disadvantage of fiber optic conductors is that they must be physically installed, the fact that an enormous amount of information can be transmitted over the fiber optic conductors reduces the number of fiber optic conductors which must be installed. This avoids some of the problems in metropolitan areas were space for additional cables is difficult to obtain. In those circumstances where the information is communicated over long distances, fiber optic conductors are the typical medium employed for such long-haul transmissions.
Free-space atmospheric links have also been employed to communicate information optically. A free-space link extends in a line of sight path between the optical transmitter and the optical receiver. Free-space optical links have the advantage of not requiring a physical installation of conductors. Free-space optical links also offer the advantage of selectivity in eliminating sources of interference, because the optical links can be focused directly between the optical transmitters and receivers, unlike RF communications which are broadcast without directionality. Therefore, any adverse influences not present in this direct, line-of-sight path or link will not interfere with optical signals communicated.
Despite their advantages, optical-free-space links present problems. The quality and power of the optical signal transmitted depends significantly on the atmospheric conditions existing between the optical transmitter and optical receiver at the ends of the link. Rain drops, fog, snow, smoke, dust or the like in the atmosphere will refract or diffuse the optical beam, causing a reduction or attenuation in the optical power at the receiver. The length of the free-space optical link also influences the amount of power attenuation, because longer free-space links will naturally contain more atmospheric factors to potentially diffuse the optical beam than shorter links. Furthermore, optical beams naturally diverge as they travel greater distances. The resulting beam divergence reduces the amount of power available for detection. If the attenuation of the optical beam is sufficiently great, the ability to recognize the information communicated on a reliable basis is diminished, and the possibility that errors in communication will arise is elevated. Atmospheric attenuation particularly diminishes the probabilities of error-free communications at higher transmission frequencies, because atmospheric attenuation naturally occurs to a greater extent at higher optical frequencies, i.e. shorter wavelengths, than at lower optical frequencies.
One approach to reducing the adverse influences of atmospheric attenuation is to use laser beam transmissions in the free-space links at frequencies which are capable of greater penetration and less refraction or diffusion by atmospheric influences. Unfortunately, the more penetrating frequencies are sometimes also the ones which can easily damage human eyes. To maintain safety while still avoiding some of the problems from atmospheric attenuation, the amount of power which is optically transmitted at these more penetrating frequencies is substantially limited. Since the more penetrating frequencies are also subject to beam divergence, reducing the power still complicates the reliable communication of information. Consequently, the reduced power transmission levels counter-balance the benefits of the lesser atmospheric attenuation at the more penetrating frequencies. Because of the reduced power at the more penetrating frequencies, the effective length of the free-space optical links is still limited.
Furthermore, the more-penetrating, free-space optical frequencies are different from those frequencies which are typically employed to transmit information over long-haul fiber communication systems. An electro-optical conversion is required to convert the fiber link backbone transmission frequency to the free-space transmission frequency. An electro-optical conversion involves converting the higher frequency optical signals to electrical signals and back to optical signals at the more penetrating laser frequency, and vice versa. Additional equipment is required to accomplish the conversion, resulting in an increase in the cost and complexity of the terrestrial optical communications network.
In addition, electro-optical conversions also introduce the possibility that errors will be created during the conversion, particularly under the common situation of the fiber optic signal carrying information at multiple different wavelengths. Common optical detectors respond to information in a broad frequency range or wavelength band, and this broad-band response destroys the information carried at specific wavelengths. To avoid this problem and to maintain the information present in the different, specific wavelength optical signals, the optical signal must first be filtered into its different wavelength components. Thereafter each different wavelength component must be separately electro-optically converted, and then all of the separate converted components combined back into a single optical signal. The complexity of this process raises the possibilities of introducing errors in the information communicated and increases the costs of the equipment used in the terrestrial optical communication network.
Electro-optical conversion has also been used to amplify the light signals conducted over fiber optic cables. The light signals conducted over fiber cables will attenuate, and it is periodically necessary to amplify those signals in order to maintain signal strength. Recently however, erbium doped fiber amplifiers (ERDAS) have been developed to amplify the light signals optically, without requiring electro-optical conversion, as the light signals pass through the optical fiber. ERDAs allow light to be amplified in a relatively wide wavelength band (about 30 nanometers (nm)) around a 1.55 micrometer (um) fundamental wavelength. ERDAs are of particular advantage in long haul telecommunications systems, because these systems normally operate in the 1.55 um wavelength range. The broad band amplification of ERDAs around the 1.55 um fundamental frequency allows the ERDAs to be integrated into systems using wavelength division multiplexing (WDM), resulting in the ability to communicate separate information at different wavelengths simultaneously in the same fiber. Thus, ERDAs are of particular importance and value in long haul fiber telecommunication systems because electro-optical conversions can be avoided.
It is with respect to these and other background information factors relevant to the field of terrestrial optical communications that the present invention has evolved.
The present invention advantageously addresses the needs above as well as other needs by providing a method of optical communication that comprises the steps of receiving an optical signal from a free-space link with a receive element; directing the optical signal received from the free-space link by the receive element into a fiber amplifier that is optically coupled to the receive element; optically coupling the optical signal from the fiber amplifier into a fiber optic link of a terrestrial optical communication network; and controlling a power gain of the fiber amplifier in response to the optical signal. In another version, the present invention provides an apparatus for performing these functions.
Another version of the present invention provides a method of optical communication that comprises the steps of: optically coupling a first optical signal from a fiber optic link of a terrestrial optical communication network into a fiber amplifier; amplifying the first optical signal with the fiber amplifier, directing the first optical signal through a free-space link with a beam focusing element optically coupled to the fiber amplifier; and controlling a power gain of the fiber amplifier in response to a second optical signal received over the free-space link. In another version, the present invention provides an apparatus for perfinning these functions.
Other versions of the present invention may provide methods of optical communication that comprise the above steps, except that the step of controlling a power gain may be replaced with the step of controlling a physical position of at least one of the receive element and the beam focusing element relative to an optical signal path of the free-space link in response to an optical signal.
Yet another version of the present invention provides a method of optical communication that comprises the steps of receiving a first optical signal from a free-space link with a receive element; directing the first optical signal received from the free-space link by the receive element into a doped single mode core of an optical fiber of a first fiber amplifier that is optically coupled to the receive element; optically coupling a second optical signal into a doped single mode core of an optical fiber of a second fiber amplifier; directing the second optical signal through the free-space link with a beam focusing element optically coupled to the doped single mode core of the second fiber amplifier; and controlling a physical position of at least one of the receive element and the beam focusing element relative to an optical signal path of the free-space link in response to the first optical signal. In another version, the present invention provides an apparatus for performing these functions.
One of the important improvements of the present invention relates to the seamless, all-optical integration of fiber optic links and free-space links in a terrestrial communication network without electro-optical conversion between the free-space and fiber optic links. Another improvement relates to communicating information over fiber optic links and free-space links in a terrestrial optical communication network at wavelengths which do not require frequency conversion. Another improvement relates to operating free-space optical links in a terrestrial optical communication network at laser wavelengths which are safer to human eyesight with sufficient link power margin to avoid many of adverse influences of atmospheric attenuation and divergence. Still another improvement involves operating a terrestrial optical communication network with free-space links at a fundamental wavelength which is compatible with or approximately equal to the fundamental wavelength typically used in long-haul optical communication systems. A further improvement relates to establishing free-space links in a terrestrial optical communication network which are not line-of-sight and without using optical repeaters that require electro-optical conversion. Still a further improvement relates to changing the physical orientation of the optical receivers and transmitters to maximize the link power margins of the optical beam and thereby increase the reliability of the transmission. A last specifically mentioned improvement, among others which are not specifically mentioned here, is to is to implement a relatively low cost, terrestrial, all-optical communication network by making effective use of erbium doped fiber amplifiers (ERDAs).
These and other improvements are attained by a terrestrial optical communication network which comprises a plurality of fiber optic links and free-space links between which optical signals are optically coupled by erbium doped fiber amplifiers (ERDAs), and by a method of terrestrial optical communication which comprises the steps of establishing a plurality of fiber optic links and free-space links between which optical signals are optically coupled without electro-optical conversion, preferably by ERDAS. The ERDAs optically couple the optical signals between the fiber optic and free-space links, thereby avoiding the necessity for electro-optical conversion as the optical signals transition between the free-space and fiber optic links. The relatively inexpensive ERDAs produce all-optical, relatively broad-band amplification, which avoids the necessity for electro-optical conversion. The fundamental wavelength of the ERDAs is also compatible with the fundamental wavelength used in long-haul and backbone communication systems, thereby allowing a convenient integration of the network of the present invention with those communication systems. The fundamental wavelength of the ERDAs is also safer to human eyesight.
Other improvements are attained by a terrestrial optical communication network which comprises a plurality of links between which optical signals are optically communicated by a transmitting ERDA. The transmitting ERDA an optical signal before its transmission over a link and a controller connected to the transmitting ERDA controls the optical power gain of the transmitting ERDA. The controller adjusts the optical power of the transmitted optical signal in response to the optical power of optical signals received over the link. A related method of terrestrial optical communication involves sensing the optical power of optical signals received over the link and adjusting the optical power of the optical signal transmitted over the link in response to the sensed optical power of the received optical signals.
Other preferred aspects of the power control improvements include controlling the optical power gain in response to the received power of signals at the transmitting and receiving locations, controlling the optical power gain of a receiving amplifier, transmitting power control information optically between the receiving and transmitting ends of the links, and controlling the optical gain of the transmitting and receiving ERDAs to obtain the best quality signals.
By controlling the power transmitted and received in accordance with these improvements, the disadvantage of the less penetrating wavelengths of the ERDAs is minimized by the higher power available for optical communication. Furthermore, the power control improvements obtain a better quality optical signal for communicating information.
Still other improvements are attained by a terrestrial optical communication network which comprises a plurality of links between which optical signals are optically communicated by a transceiver which transmits and receives optical signals communicated over a link and a controller connected to the transceiver to control the physical position of the transceiver relative to the optical signal path of the link in response to the optical signals received over the link. A method of terrestrial optical communication comprises the steps of connecting an adjustment mechanism to the transceiver to adjust the position of the transceiver and adjusting the position of the transceiver by controlling the adjustment mechanism. By controlling the position of the adjustment mechanism in accordance with these improvements, a better quality optical signal for communicating information is assured.
A more complete appreciation of the present invention and its scope can be obtained by reference to the accompanying drawings, which are briefly summarized below, to the following detailed description of presently preferred embodiments of the invention, and to the appended claims.